The activating and stabilizing effects of inorganic pyrophosphate, tripoly-phosphate and nucleoside triphosphates on firefly luciferase biolumines-cence were studied. The results obtained show that those effects are a consequence of the luciferase-catalyzed splitting of dehydroluciferyl-adenyl-ate, a powerful inhibitor formed as a side product in the course of the bio-luminescence reaction.